1. Field of the Invention
The present invention relates to a private branch exchange, a VoIP gateway unit and a private branch exchange system, and more particularly, it relates to a private branch exchange, a VoIP gateway unit and a private branch exchange system each capable of coping with a nuisance call.
2. Description of the Background Art
An Internet telephone terminal unit capable of coping with a nuisance call is known in general, as disclosed in Japanese Patent Laying-Open No. 2005-260456, for example.
The aforementioned Japanese Patent Laying-Open No. 2005-260456 discloses an Internet telephone terminal unit writing information related to a caller making a call disconnected within a prescribed time from incoming in a filter table as the so-called one-ring call (nuisance call) (call disconnected after only short-time ringing) and not to drive a tone ringer for an incoming call received from the caller whose information is written in the filter table. Thus, the Internet telephone terminal unit is so formed as to cope with a one-ring call (nuisance call) by inhibiting a caller having ever made a one-ring call (nuisance call) from repeatedly making the one-ring call (nuisance call).
However, the Internet telephone terminal unit according to the aforementioned Japanese Patent Laying-Open No. 2005-260456, capable of coping with a one-ring call (nuisance call), cannot cope with a nuisance call put through thereto without being disconnected after only short-time ringing. A private branch exchange connecting a telephone terminal unit employed in a company or an office to a public telephone network, a VoIP gateway unit connecting a telephone terminal unit in an IP network to the public telephone network and a private branch exchange system including the private branch exchange and the VoIP gateway are known in general. When applied to the telephone terminal unit connected to the public telephone network through the private branch exchange or the VoIP gateway unit, the Internet telephone terminal unit according to the aforementioned Japanese Patent Laying-Open No. 2005-260456 cannot cope with the aforementioned nuisance call put through thereto without being disconnected after only short-time ringing.